This invention relates to reconfigurable light-weight reflectors of radiant energy, such as optical reflectors, microwave energy reflectors and the like, that have application in space satellites and, more particularly, to a very light-weight reflector of unique structure requiring minimal electronics and post deployment control of reflector shape.
Reflectors have long been used to reflect radiant energy, energy traveling as wave motion, which encompasses visible light, infra red light, and radio and microwave frequency energy. Those reflectors have been applied in communication systems, detection and radiant energy exploration systems, and the like for redirecting incident radiant energy, such as found in antennas. Reconfigurable large aperture reflectors, twenty-five meters or larger in diameter, have particular application in space borne optical and radar equipment for the exploration of distant galaxies and the Earth. The large area of the reflector permits imaging of faint targets and coverage of larger areas of the Earth. As for any reflector intended for space borne application, a large aperture reflector must be able to fit inside a launch vehicle in a packed or folded condition, often within a volume and space prescribed by the space vehicle manufacturer, and deploy on orbit with repeatability and reliability. Launch weight is an additional factor and a very important one because of the impact on deployment cost. The greater the weight, the greater amount of fuel is required for the space vehicle to lift off and achieve orbit. The reflector must be light enough in weight for launch into orbit; and the lighter the weight of the reflector, the better.
The current state of the art in lightweight flight optics is in the four meter LAMP telescope that is believed to possess a reflector of an areal density of approximately 40 kg/m2. The patented AstroMesh(trademark) reflector, described in patent U.S. Pat. No. 5,680,145 and at the IEEE Antennas and Propagation Society (APS) International Symposium, July 1999, demonstrated areal densities of 0.5 kg/m2, which serves as the present minimum of areal density for such reflectors. Despite those achievements, a demand and ready market awaits reflectors of even lower areal density if and when created.
The present large aperture space borne antennas of 25 meters diameter and larger are fabricated with lightweight compliant systems, such as thin film supporting a reflective surface, to permit small stowage volumes. Those reflectors can require distributed actuation and control for deployment and subsequent precision control of surface shape under a variety of space conditions, all of which requires electronic hardware. That actuation and control hardware is both complex and contributes significantly to the weight of the antenna system. For that reason alone any reduction of weight in actuation and control hardware for the deployable reflector is also desirable. Further, lessening the weight of the electronic control hardware allows reduction in manufacturing tolerances of the membranes and that is also desirable.
Reconfiguration of a stowed reconfigurable reflector to a fully deployed condition requires a deploying mechanism, referred to herein as an actuator. On command, the actuator mechanically moves the thin film reflector from a folded condition in which stowed to the deployed condition in which the thin film reflective surface of the reflector is spread over a large area. One known small size actuator that, until the present invention, has not been successfully combined as a component of a reconfigurable reflector, is a shape memory device. One form of the shape memory device comprises a shape memory alloy that is thermally actuated and that device has been employed heretofore as an actuator in switches and as pin pullers and the like, outside of reconfigurable reflectors.
The shape memory device contains a shape memory alloy film (as example, in the form of a strip or block) that possesses a xe2x80x9cmemorizedxe2x80x9d physical shape created on fabrication. That initial shape can be manually changed, reconfigured, from the memorized physical shape to a different one. Once manually reshaped, the alloy remains in the changed shape until the temperature of the alloy is raised above a transition or transformation temperature, characteristic to the alloy, wherein the alloy undergoes a metallurgical phase change from the martensite phase to the austenite. As example a strip of alloy that is fabricated as a flat strip, the memorized shape, may be rolled up into a coil, then remains as a coil until the temperature is raised above the characteristic transformation temperature. When the transformation temperature is attained, the strip uncoils on its own using stored energy and returns to the memorized flat shape. The physical transition occurs very fast, and is said to occur at a speed equal to about one-third the velocity of sound. The foregoing ability of a metal alloy to recover a particular surface shape when heated above a certain temperature alloys is referred to as a shape memory effect (xe2x80x9cSMExe2x80x9d).
One known composition for the shape memory material is an alloy formed of the metals nickel and titanium (NiTi). One form of that alloy is a specific type of wire developed at the U.S. Naval Ordnance Laboratory, referred to as Nitinol. Such material exhibits a shape memory effect (xe2x80x9cSMExe2x80x9d) only over a very limited range of proportions of the two metal components of the alloy. At one end of the range of SME the composition is formed of titanium rich material, as example, 51% titanium and 49% nickel. At the other end of the range of SME the composition is formed of a nickel rich material, as example, 51% nickel and 49% titanium. Each specific composition within the general range exhibits varied shape memory characteristics, identified by a varying transformation temperature. Outside the composition range, the alloy lacks SME. Other metal alloys, such as nickel titanium copper (NiTiCu), nickel titanium palladium (NiTiPd), and gold copper (AuCu), as example, also exhibit the SME and may be found in the technical literature.
NiTi exhibits the ability to recover large deformations, for example 10% strain, with a thermally induced phase transformation from martensite to austenite phases. Deforming the material at room temperature in the martensite phase causes a permanent deformation similar to the deformation of a copper wire, a ductile metal, when wound around one""s finger. Unlike copper, however, when the NiTi is heated above the critical temperature (Af), about 100xc2x0 C. for a 51% Ti 49% Ni composition, the NiTi alloy transforms to the austenite phase and immediately springs back to the original (e.g. xe2x80x9cmemorizedxe2x80x9d) shape. The large strains (e.g. distortions) of ten percent provided by the NiTi alloy (and other shape memory alloys) are adequate to fully deploy a membrane structure.
Once deployed in space, the reflector, typically, remains deployed. Usually, there""s no need to, as example, to re-stow the reflector, and no thought is given to that end. As an advantage, the present invention introduces that additional capability. Further, although one might not consider re-stowing a space borne reflector, the present inventors recognize that other types of reconfiguration of a reflector may be desirable following deployment. As example, when a reflector is deployed in outer space, the reflector should exist in the three-dimensional geometric shape intended by the designer of the reflector, such as a parabola. Due to a mechanical fault or for reasons unknown, a surface portion of the reflector may be dented in or otherwise fail to assume the correct configuration or shape, whereby the performance of the reflector is adversely affected. Since the reflector is essentially inaccessible, it is not easy to change that incorrect configuration to the correct one. For that purpose, one may incorporate remotely controlled electromechanical actuators and/or the like at strategic locations on the reflector and make adjustments by remote control. But, for one, such actuators would greatly increase the weight of the reflector, are complicated to pack in with a stowed reflector and themselves introduce a series of devices that may be a source to other forms of failure. As an advantage, an aspect of the present invention addresses such adjustments of reflector configuration and provides for such reconfiguration.
A feature of some space borne reflectors is that the position of the reflector is set and may be changed by an electrical motor that is remotely controlled. By remote control the ground station operator is able to change the direction of the reflector, whereby the reflector may direct incident radiant energy to one location and then at other times redirect the incident radiant energy to a second location, a feature that is useful to some applications. Motors and the rotatable reflector posts for that function are generally heavy and take up space. Another advantage of the present invention is to serve a like function without requiring motors and rotatable posts further reducing the weight of the reflector.
Accordingly, a principal object of the present invention is to produce deployable reflectors that posses areal densities lower than 0.5 kg/m2.
A further object of the invention is to advance the state of areal density of a reconfigurable large aperture reflector to an areal density of less than 0.1 kg/m2 , and even as low as 0.01 Kg/m2.
A still further object of the invention is to reduce the weight of space borne configurable large aperture antenna systems by reducing or minimizing the amount of hardware necessary to control deployment and post deployment reflector shape.
Another object of the invention is to permit a previously deployed space borne reflector to automatically be reconfigured.
Still another object of the invention is to correct any distortions in the shape of a reflector, that is, reconfigure or xe2x80x9ctweakxe2x80x9d the shape, without using electromagnetic actuators.
Yet another object of the invention is to change the direction of reflection of incident radiant energy by a reflector without the use of electric motors and/or electromagnetic actuators.
Still another object of the invention is to provide a process for fabricating a reconfigurable reflector that possesses the capability of being changed from a stowed condition to a deployed condition and vice-versa.
And, an ancillary object of the invention is to provide a process for manufacturing shape memory alloy reflectors that possess a two-way shape memory effect.
In accordance with the invention, a reconfigurable reflector is formed of a laminate of thin film material and a thin layer of a shape memory alloy, with the laminate containing a reflective surface. The laminate is configured in one geometry while the reflector is in the stowed condition and the temperature remains below the transformation temperature of the shape memory alloy, and transforms or deploys to a second deployed geometry, in which the reflective surface of the reflector is fully expanded when the temperature is at or above that transformation temperature. Means are provided to heat the reflector to at least the transformation temperature when reflector deployment is desired. In accordance with a specific aspect to the invention, the latter means comprises an electric heater.
In accordance with another aspect to the invention, the foregoing shape memory material is of the two-way type, wherein withdrawal of heating results in the reflector returning to the stowed condition once the temperature of the reflector falls below the transformation temperature. For that two-way type shape memory material, the layer of shape memory material comprises a compositionally graded layer of shape metal alloy, which in the preferred embodiment ranges in grade from 49% nickel and 51% titanium to 51% nickel and 49% titanium and, in thickness, in the range of five and twenty microns.
Further in accordance with the invention, a new method is presented for constructing a reflector that includes draping a reflective membrane onto a support surface inside a sputtering chamber, that support surface defining the geometry of the reflector in the deployed condition, with a reflective surface of said reflective membrane being oriented face down against said support surface; and then sputtering a shape memory alloy onto the outer surface of said reflective membrane to cover said surface and produce a laminate assembly, the outer surface of said reflective membrane being opposite to said reflective surface.
In accordance with a specific aspect of the foregoing method the sputtering process accomplishes compositional grading of the shape memory alloy, specifically a two-metal alloy that exhibits the shape memory effect. The procedure for obtaining that grading during sputtering is to heat the metal alloy that supplies the metal atoms to the plasma to a predetermined temperature and thereafter increment that heating (and temperature) in steps at defined intervals. Suitably, the initial temperature is selected so that the alloy composition sputtered onto the membrane falls at one end of the range of shape memory effect of the alloy and the final temperature is selected so that the sputtered composition of the outer surface of the shape memory alloy is at the other end of that range.
The foregoing and additional objects and advantages of the invention, together with the structure characteristic thereof, which were only briefly summarized in the foregoing passages, will become more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment of the invention, which follows in this specification, taken together with the illustrations thereof presented in the accompanying drawings.